Proton-translocating adenosine triphosphatases (proton ATPases) are a large class of membrane-proteins that use the energy of ATP hydrolysis to generate an electrochemical proton gradient across a membrane. The resultant gradient may be used to transport other ions across the membrane (Na.sup.+, K.sup.+, or Cl.sup.-) or to maintain an acidic environment important to the function of many cellular vesicles (Mellman, I. et al. (1986) Ann. Rev. Biochem. 55:663-700). Proton ATPases are further subdivided into the mitochondrial F-ATPases, the plasma membrane ATPases, and the vacuolar ATPases.
The vacuolar proton ATPases (vp-ATPases) provide most of the energy required for transport processes in the vacuolar system in eukaryotic cells. vp-ATPases establish and maintain an acidic pH within various vesicles involved in the processes of endocytosis and exocytosis. Such vesicles include phagosomes, lysosomes, endosomes, and secretory vesicles. Endocytosis is the process in cells of internalizing nutrients, solutes or small particles (pinocytosis), or large particles such as internalized receptors, viruses, bacteria, or bacterial toxins (phagocytosis). Exocytosis is the process of transporting molecules to the cell surface. Exocytosis facilitates the placement or localization of membrane-bound receptors or other membrane proteins and the secretion of hormones, neurotransmitters, digestive enzymes, and wastes. Endocytosis and exocytosis are fundamental to the function of all types of cells.
Alterations in both endocytosis and exocytosis play a role in a variety of disorders. For example, synaptic vesicles play a major role in neural transmission at nerve terminals through storage and controlled release of neurotransmitters. Neurotransmitter uptake into synaptic vesicles is driven by the electrochemical proton gradient generated by vp-ATPase. Inactivation of vp-ATPase has been shown to inhibit glutamate uptake by synaptic vesicles, decreasing neurotransmitter release during episodes of oxidative stress or in response to second messenger signaling. Additionally, inactivation of vp-ATPase has been shown to trigger apoptosis in a variety of immortalized and primary cell lines. Activation of vp-ATPases has been found to delay apoptosis (Wang, Y. et al. (1998) J. Neurochem. 70:646-652; Nishihara, T. et al. (1995) Biochem. Biophys. Res. Commun. 212: 255-262; and Niessen, H. et al. (1997) Blood 90: 4598-4601.)
The vp-ATPases and the F-ATPases, which function in ATP synthesis and hydrolysis in mitochondria, are related in both their subunit structure and evolutionary origin. Both contain distinct catalytic and membrane sectors, and each sector contains multiple subunits. The catalytic sector of vp-ATPase consists of five subunits designated A (72 kDa), B (57 kDa), C (41 kDa), D (34 kDa), and E (33 kDa) (Nelson, H. et al. (1995) Proc. Natl. Acad. Sci. 92:497-501). Three subunits, AC115, AC39, and a proteolipid component, have been identified in the membrane sector of vp-ATPase from various sources. The proteolipid subunit has been implicated in the mechanism of energy transfer in the enzyme. The membrane sector has several functions including proton conduction across the membrane, energy coupling with the catalytic sector, communication with the lumen, and modulation of enzyme activity. Mutational studies in yeast have shown that, while the membrane sector may be assembled independently of the catalytic sector, assembly of the catalytic sector is absolutely dependent on previous assembly of the membrane sector (Noumi, T. et al. (1991) Proc. Natl. Acad. Sci. 88: 1938-42; Ludwig, J. et al. (1998) J. Biol. Chem. 273: 10939-10947; and Supekova, L. et al. (1996)199:1147-1156). Thus, expression and assembly of the membrane sector subunits control the overall activity of the enzyme complex.
The discovery of new vacuolar proton ATPase subunits and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of cell proliferative and neurological disorders.